Carrier wave signaling system



A. D. RING 32,312

CARRIER WAVE SIGNALING SYSTEM Filed May 19, 1933 2 Sheets-Sheet l Inventor": Andrew D. Ring,

- His Attorne g.

F850 25, 193%. gym N 2,@3231l2 CARRIER WAVE SIGNALING SYSTEM Filed May 19, 1955 2 Sheets-Shet 2 Fig. 2.

007F117 CURRENT KIWA Inventor: Andrew D Ring,

MWLX w His A'ljl'iorney.

Patented Feb. 25, 1936 UNITED STATES CARRIER. 'WAVE SIGNALING SYSTEM m d-rew .D. Ring, Washington, .D. (3., assignor to General Electric Company, a corporation of New York Application 'May 19, 1933, Serial No. 671357 8 Claims.

My invention relates to carrier wave signaling systems, and more particularly to modulating systems therefor. It has for one of 'its obiects to provide a method and means whereby a sub stan tially uniform percentage modulation of the transmitted carrier wave produced irrespective of the intensity of the signals to be transmitted.

In modulation systems of the prior art, the

modulated high frequency elecin'omdtive force has -a percentage modulation varying with the intensity of the signals transmitted. During the transmission of weak signals the transmitted energy is modulated only to a small extent and thus has a large unmodulated energy component,

that is, the percentage modulation is small. During the transmission of strong signals the percentage modulation is correspondingly increased, and the unmodulated component reduced. The

unmodulated component of the transmit-ted energy has no part in the production of signals in any demodulating signal reproducing apparatus, and therefore represents wasted energy, the intensity of the reproduced signal being dependent upon the percentage modulation.

. One of the objects of my invention is to reduce, or eliminate this wasted energy.

A further object of my invention is "to provide a method and means whereby the carrier Wave transmitted is continuously modulated with a constant percentage modulation irrespective of the intensity of the transmitted signals.

Still a further obiec't of my invention is to provide such a system in which the constant percentage modulation may be one hundred per cent.

The novel features which believe "to be characteristic of my invention are set forth with particu'larityin the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof may best he understood by reference to the following descrip tion taken in connection with the accompanying drawings in which Fig. '1 represents an embodiment of invention, and Figs. and 3 illustrate certain characteristics pertaining to its operation. 7

Referring to Fig. '1, I have shown in the uppe portion thereof the modulating stages of a carrier wave "transmitter, and in the lower portion the means whereby the modulation is effected. The carrier wave produced by a source of oscillations I is amplified by an electron discharge amplifier 2- and a pull electron discharge am pli fit-rr 3 ithe output of which is coupled to a load circuit "4 which may lead to the in-put circuit of subsequent stages of the transmitter, or to an antenna, or transmission -line. While the amplifiers 2 and 3 "may he of any suitable construction they are indicated as comprising electron discharge devices of the ordinary threeelement type connected "in circuits in which the capacity between the anode and grid of each device is neutralized 'by a neutralizing condenser indicated, in each case, by the reference numeral 5. The output of the amplifier 2 tuned 'by means of a condenser B, and the input -to the discharge devices of the amplifier 3 is tuned vby means of condensers and inductance ll. Simi- 15 larly the output of this amplifier is tuned by condensers -8. Each of the discharge devices of the amplifier -'3 has a loading resistance 9, l0 connected between its grid and the cathode through a common condenser 12, the purpose 20 of these resistances being so to load the output of the amplifier 2 as to reduce the distortion produced by the drawing of power by the grids of the amplifier 3 during periods when these grids are "positive. 2

The grids o f the discharge devices in the ampliher 3 are biased negatively with respect to their cathodes by means of a source of potential '13, shown in the lower portion of the figure. The bias circuit extends from the grids through the 30 opposite portions of the input tuning inductance I I, radio frequency choke coil :14, conductor 15, resistance 16, and battery 13 to ground, and thence through ground to the cathodes through conductor H. 35

In accordance with my invention the oscillations produced by the source I are modulated in the usual way in the output circuit of the ampliher 2. 'They are then supplied to the input circuit of the amplifier 3. The bias voltage on the 40 grid of the amplifier 3 is so adjusted and controlled in response to the intensity of the signal curren'tsthat any constant component of the-output carrier wave is reduced or eliminated. This is effected by producing a voltage upon the re- 45 sistance having a value which varies with the envelope of the signal wave and causing this voltage to oppose the constant bias produced by the source whereby the total :b'ia-s voltage supplied to the grids of the discharge device of the 50 amplifier 3 is varied in accordance with the :envelope of the signal wave.

To explain more in detail the means whereby this is efiected the source of signal waves .is represented in the drawings as an ordinary telephone 55 transmitter or microphone l8 the output signal currents of which are amplified by amplifier l9 and supplied to the grid of a modulator tube 20. The anode of this tube is connected to the oathode through the usual modulating reactor 2| and voltage source 22. This anode is also connected to the anode of the amplifier 2 through a radio frequency choke coil 23. Thus the discharge device 20 is connected to the discharge device of amplifier 2 in the ordinary manner for anode modulation. Signal currents from the anode of the discharge device 20 are also amplified in amplifier 20 and supplied through condensers 24 and 25, and transformer 26 to a full wave rectifier 21 in which the signal currents are rectified and supplied through a filter 28, 29,

to the resistance l6.

While very satisfactory results have been obtained with the arrangement as thus described, it is preferred that the rectifier 21 be employed only to rectify the higher frequency portion of the signal currents and that additional means be provided to rectify the lower frequency portion of the signal currents. To accomplish this, signal currents from the output of the amplifier 20' are also supplied through the condenser 24, an inductance 30, and transformer 3| to a frequency doubling device 32. This frequency doubling device comprises two two-element electron discharge devices having their anodes connected respectively to the opposite terminals of the secondary winding of transformer 3| and their cathodes connected together and to the midpoint on the secondary winding of the transformer 3| through a reactance 33 and source of electromotive force 34. As thus arranged, onehalf cycle of the signal wave passes through one of the discharge devices and the other half cycle passes through the opposite discharge device whereby the electromotive force produced across inductance 33 is of double the frequency of that appearing on the secondary winding of transformer 3|. This electromotive force is supplied through transformer 35 to a full wave rectifier 36 which is connected to supply a unidirectional electromotive force to resistance I6 through a smoothing network or filter, comprising smoothing condenser 31 and reactance 38.

The reactance 39 and condenser 25 are so relatively proportioned that only the higher frequencies traverse the condenser 25 and only the lower frequencies traverse the transformer 3|. Both systems of rectifiers, however, operating together serve torectify the entire signal frequency range and to produce a relatively smooth unidirectional electromotive force upon resistance I6, the magnitude of which varies with the envelope of the signal wave.

The operation of the system described may now be better understood by reference to the characteristics shown in Fig. 2. The characteristics of this figure are merely presented by way of illustrating in a simple way the principle of operation of the device. In this figure the curve 39 may be considered as presenting the relation between the carrier wave current produced in the output circuit 4 and the voltage supplied to the grid of the discharge device 3. The distance 40 between the line 42 and the zero grid bias axisrepresents the magnitude of unidirectional electromotive force supplied to the grid of the discharge device by the battery |3 when no signal voltage is being produced by the transmitter l8. The carrier wave supplied to the grids during this time then oscillates about the line 42 and may have an amplitude equal to the distance between lines 42 and 4| or just insufficient to cause a flow of current in the anodes of amplifier 3 and, hence, just insufficient to cause any oscillatory electromotive force to be produced in the output circuit 4.

Now, assuming that a signal wave of varying intensity represented by the curve 43 of Fig. 2 is impressed upon the amplifier 20. This signal wave will cause the envelope of oscillations supplied to the grids of amplifier 3 to be modulated in accordance with the form of the signal wave. Also through action of the automatic bias arrangement described, including resistance IS, the bias voltage supplied to the grids of the discharge devices: of amplifier 3 will be reduced in accordance with the envelope of this signal wave. This envelope is indicated in the figure by the curve 44. This means that the bias voltage about which the, carrier wave supplied to the grid oscillates 'willl be moved to the right by an amount corresponding to the envelope 44. This is indicated at the left side of the figure by the portion 45, of the line 42. Were it not for the fact that these oscillations are modulated in the output circuit of amplifier 2 their amplitude would be constant during the interval of the signal wave and would be represented in the figure by the distance between ,the portions 45 and 46 of the lines 42 and 4|, respectively. Since that portion of these oscillations lying between the line 4| and curve 48 is at the right of the point of curve 39 corresponding to zero output an impulse of oscillatory current having the form of curve 46 would be produced in the output circuit 4. However, since the amplitude of these oscillations is modulated in the output of amplifier 2 by the signal Wave 43 the envelope of the oscillations supplied to the grid will be represented by the curve. 41 is at the right of the point of curve 39 corresponding to zero current in the output circuit 4. This means that for every value of input electromotive force at the right of line 4| a corresponding value of output current is produced in the circuit 4. The envelope of the output carrier wave electromotive force will then have the form indicated in the figure by the curve 48. It will be noted with reference to the curve 48 that the modulation depth of the carrier wave represented thereby extends, in each instance, to the zero axis, or that the carrierwave produced in the output circuit is continuously modulated 100% notwithstanding the variations in amplitude of the signal wave 43. Demodulation of the carrier, the envelope of which is represented by curve 48 will cause the signal wave 43 to be reproduced.

It will thus be seen that with the device as thus disclosed the carrier wave produced in the output circuit 4 and which maybe radiated into space for reception in remote receivers is continuously modulated 100% irrespective of the intensity of the signal wave transmitted. That is, any constant component of, the modulated carrier wave produced during periods of low sig nal intensity is completely removed. This is an important result since this constant component represents a large amountlof radio frequency energy which is radiated into space and which is ineffective to produce signal in remote receivers. It therefore represents wasted energy. The system disclosed produces no radiation of carrier energy except when signal currents are to be transmitted as when the operator speaks into The whole of curve y to the grids.

the transmitter 18 and then thexcarrier wave transmitted varies :in (amplitude in accordance with the instantaneous intensity of the signal to be transmitted and maintains :a 100% modulation.

It .is important in the operation of .my invention that the rectifier whereby the automatic bias is obtained be one capable of rectifying high frequency currents since in this way the device is adapted to respond to extremely rapid increases .in intensity of the .signal electromotive force. At the same time it .must be capable of producing proper rectification of the low frequency components of the signal currents. For this reason .separate rectifiers are employed for the high and low frequencies respectively. In this way; in the event of a sudden rise in signal intensity the automatic bias is first applied by the high frequency rectifier. Were no such rectifier employed sufficient interval of time would be required for this bias to build up to cause the initial portion of the signal to be lost. At the same time if the .high frequency rectifier were depended upon to supply the entire bias insufficient smoothing of the low frequency signal components, and consequent undesired double modulation, would result. Best operation is therefore obtained with the arrangement above described.

It may be desirable, however, continuously to transmit a certain constant component of carrier wave energy and to modulate this energy to a variable extent dependent upon the intensity of the signal wave, but without permitting the minimum intensity of the modulated electromotive force normally to fall below the intensities of said constant component. This may be desirable, for example, where the lower portion of the curve .39 is curved sufiiciently to render it undesirable to operate normally upon this portion of the curve. It is desirable, however, that the minimum intensity of the carrier electromotive force be caused to drop below the intensity of the normal constant component when very strong signals are to be transmitted, since were this not the case the apparatus would be limited in its capacity to transmit strong signals. In accordance with my invention these results may be efiected.

This manner of operation of the invention may be obtained either by varying the constant portion of the bias voltage supplied to the grids of the discharge devices of amplifier 3 or by varying the amplitude of oscillations supplied For this reason the source 13 and also the carrier wave source I have been indicated as variable by the arrows drawn therethrough. In order that the intensity of the carrier may drop below the intensity of the normal constant component upon the transmission of strong signals, I also provide an electron discharge device 49 connected between the conductor I5 and a suitable point upon the source [3. This discharge device may be of any suitable type but preferably is one of the hot cathode vapor electric type in which current passes only.

when the voltage between the anode and cathode exceeds a predetermined value. As so connected, the bias voltage upon the amplifier is prevented from being reduced through action of the automatic bias control means by more than a predetermined amount after which the bias voltage remains constant.

The action of the system during this type of operation is better shown by the characteristics of Fig. :3. This figure :is similar to 2 with the exception that the carrier wave is shown .as one having a greater intensity when unmodulated. This is represented in the drawings by'the greater distance between the lines =41 and 4.2 and by the that the line 411 is drawn at the right :of the point of curve 39 icorrespnnding to :zero output.

means, therefore, that there is a constant output during periods of no modulation. This output is represented by the portions 58 of the curve 48 of the drawings. The signal wave :43 is :shownin figure :as' one theemplitude of which gradually increases to a maximum value at the point 5:], .and :as having an envelope indicated by the curve 52. The bias voltage about which the carrier wave oscillates is shown as being shifted to the right, as indicated at 45,, in accordance with envelope of the signal wave to a value of bias voltage at which the :discharge device '49 becomes conducting. Thereafter, and during the time the signal voltage .is of intensity greater than that required to :cause the tube 49 to become conductive, the bias voltage remains constant, as indicated at 53, and is reduced only when the intensity of the signal wave is again reduced. The curve 54 represents the envelope of the high frequency voltage supplied to the grids of the amplifier 3. All points of this curve are either coincident with or lie at the right of the line 4| with the exception of the peak 55 corresponding to the maximum signal wave peak 5| which, due to the fact that the bias voltage is not, at this portion of the signal wave, shifted in correspondence with the envelope of the signal wave, falls to the left of the line H. The carrier wave energy then produced in the output circuit is represented by the curve 48. It will be seen that all of the points of minimum intensity of this carrier wave correspond to intensities of the carrier'equal to the normal unmodulated carrier intensity with the exception of the minimum point 58 at which the carrier intensity is less than the normal intensity. This is of course due to the fact that the bias voltage at this point was not sufficiently shifted to maintain the constant value of intensity at this minimum point. This effect is desirable, however, owing to the fact that had the bias been shifted over the region corresponding to the flat portion 53 in accordance with the envelope 52 of the signal wave then the peaks 56 and. 51 of the output wave 48 would have exceeded the knee of the curve 39 with the result that distortion would have been produced. Thus by use of the limiting tube 49 the important result is obtained that a constant unmodulated component may be maintained during the transmission of the weaker signals while at the same time the capacity of the apparatus for transmitting strong signals without distortion is not reduced.

It will be readily understood that my invention possesses numerous advantages particularly in that the total amount of high frequency energy radiated is greatly reduced thereby enabling the transmitting system to operate with greater eificiency than has heretofore been the case. Interference between stations is substantially reduced and also the transmission of ground noise during periods when no signal currents are being transmitted may be avoided.

While I have shown only a single embodiment of my invention it will of course be understood that I do not wish to be limited thereto since many modifications may be made both in the circuit arrangement and in the instrumentalities employed, and I contemplate by the appended claims to cover any? such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, an electron discharge device having on input circuit, a grid and an output circuit, means to supply carrier wave oscillations to said input circuit, means to supply a fixed bias to said grid of such value that said carrier wave oscillations are reproduced with relatively small intensity, a source of signal waves, and means to vary the bias supplied to said grid in a direction to increase the intensity of said carrier wave reproduced in said output circuit in accordance with the envelope of said signal waves, means whereby the carrier wave is modulated with the form of said signal wave, whereby the constant component of the modulated carrier wave is limited to said relatively small intensity and means whereby when the envelope of said signal waves exceeds a predetermined amplitude said constant component of the transmitted carrier becomes modulated.

2. The method of modulating a carrier wave which includes maintaining constant minimum intensities of the modulated energy for all signal intensities less than a certain predetermined value and reducing the minimum intensities of the modulated energy for signals having intensities greater that said predetermined value.

3. The method of operating an electron discharge amplifier having a grid which includes supplying to said grid a modulated carrier wave, and supplying a bias voltage to said grid varying in such a way that all minimum intensities of the amplified modulated carrier wave are equal except those minima which occur during intervals when the envelope of the carrier wave exceeds a predetermined amplitude and then so controlling said bias voltage that the minima of the carrier are of lesser amplitude than said equal intensity.

4. The method of operating an electron discharge amplifier having a grid which includes supplying to said grid a modulated carrier wave, and supplying a bias voltage to said grid varying in such a way that all minimum intensities of the amplified modulated carrier wave, corresponding to intensities of the signal wave with which the carrier is modulated, less than a predetermined value are equal and minimum intensities of the modulated carrier wave corresponding to signal wave intensities exceeding said predetermined value are of less intensity than said equal value.

5. In combination, an electron discharge device having a grid, and an output circuit, means to supply to said grid a carrier wave and the components resulting from modulation of said carrier wave with a signal wave of varying intensities, means to supply a bias voltage to said amplifier varying in accordance with the envelope of the signal wave with which said carrier wave is modulated, and means to maintain said bias voltage constant during intervals when the intensity of the signal wave exceeds a predetermined value, said predetermined value being within the normal range of signal intensities to be transmitted.

6. In a modulation control system'for audio modulated carrier wave transmitters, the method which includes separately rectifying the high and low frequency components of the audio signal,

combining the potentials resulting from such rectification and controlling the percentage modulation of the transmitter in accordance with the combined potential, whereby said percentage modulation is subject to rapid control corresponding to the high frequency components of the audio signal and undesired double modulation at the low frequency components of the audio signals is avoided.

7. In combination, a carrier wave electron discharge amplifier having a grid, means to supply a carrier wave to be amplified tosaid grid, a source of signal currents, means to modulate said carrier wave with said signal currents, means to rectify said signal currents and to supply a bias potential to said grid varying in accordance with the resulting rectified potential, and means operative in response to signals of greater than a predetermined intensity but within the range of signal intensities to be transmitted to limit said bias voltage supplied to said grid by said rectifying means to a substantially constant value equal to the bias voltage supplied when said-signals have said predetermined intensity.

8. In combination, a carrier Wave electron discharge amplifier having a grid, means to supply a carrier wave to be amplified to said grid, a source of signal currents, means to modulate said carrier wave with said signal currents, means to rectify said signal currents and to supply a bias potential to said grid varying in accordance with the resulting rectified potential, and an electron discharge device connected in shunt with the output of said rectifying means, said discharge device being conductive only when said rectified potential exceeds a predetermined value.

ANDREW D. RING. 

